For the production of hydrocarbons contained in a subsurface formation, often less than one half of the existing hydrocarbons are recovered by primary means i.e. using the natural pressure of the gas or liquids contained in the subsurface formation. To complete primary recovery, secondary recovery can be performed in which production comprises the injection of water optionally with the addition of synthetic or natural molecules (polymers, alkalis, surfactant compounds . . . ).
To optimize the recovery of hydrocarbons it is possible to conduct tracer campaigns of the reservoir. Tracers are added to the injected fluid (water or gas). The fluid travels through the reservoir. The tracers are produced together with the producted fluid (water, gas). Measurement of the time separating the injection of a tracer compound and its arrival at the production point allows the determination, inter alia, of the scanned volume of the reservoir. This is of importance when assessing expected production rates and for determining the quantity of water and/or chemical additives to be injected into the subsurface formation.
The injection of a plurality of tracer compounds allows the conducting of simultaneous analyses (for example when several injection wells are used) or of successive tracer tests over time. The changes in the quantity of different tracer compounds recovered as a function of time or of the volume of producted fluid can therefore be used to obtain a complete mapping of the flows in the subsurface formation, and for example to detect flow aberrations due to pressure differentials in the subsurface formation which may distort performance levels.
Documents U.S. Pat. No. 5,246,860, WO 02/095189 and WO 2006/047478 provide examples of methods for marking injection fluids.
At the current time, the method which is most often used is gas chromatography associated with mass spectrometry (which may or may not be tandem). However this method is very cumbersome to implement. In particular it requires a complex preparation of the samples, with a chemical derivatization of the tracer compounds necessary to transform them into gas form. Part of the tracer compounds is lost during this step. In addition, the analysis itself is time-consuming (about 60 minutes).
There is therefore a true need to provide a novel detection and analysis method for tracer compounds that is simpler, quicker and preferably having more or less the same efficiency as current methods in particular gas chromatography coupled with mass spectrometry.